1. Field of the Invention
This invention relates to a lid for use in packaging an electronic device, and in particular to a cap-shaped lid which is suitable for use with a plate-shaped base and which has a solder layer on its inner surface. The present invention also relates to a method of manufacturing such a lid.
2. Description of the Related Art
Minute electronic parts such as SAW (surface acoustic wave) filters are generally in the form of package which comprises a box-shaped ceramic base receiving an electronic device such as a surface acoustic wave device and a plate-shaped lid sealed to the top end surface of the box-shaped base. The lid is typically made of a material such as Kovar (a Co—Ni—Fe alloy) or Alloy 42 (42 Ni—Fe), which has a coefficient of thermal expansion close to that of a ceramic. A solder layer is formed on one side of the lid, and a layer of Au or other material which is easily wet by molten solder is provided on the top end surface of the base which is to be soldered to the lid. After the electronic device is disposed inside the box-shaped base, the lid is placed atop the base with the side of the lid having the solder layer formed thereon contacting the base. The base and the lid are then heated in a heating device such as a reflow furnace to melt the solder layer on the lid and thereby solder the lid to the base.
Since the base is box-shaped having side walls, the side walls of the base can interfere with the installation of the electronic device on the bottom surface of the base. The side walls are also a bar to reduction in size. Furthermore, the dies required to form a ceramic into the shape of a box are expensive, and a great amount of time is required for shaping operations.
In light of these problems with respect to the ease of manufacture and economy of packages having a box-shaped base, recently, a simple flat plate-shaped base made of a ceramic has come to be used in place of a box-shaped base for packages of minute electronic parts such as SAW filters. A plate-shaped base is easier and less expensive to manufacture than a box-shaped base, and it is easier to mount an electronic device such as a surface acoustic wave device on a plate-shaped base than on a box-shaped base because of the absence of side walls.
A plate-shaped lid such as one used with a conventional box-shaped base cannot be used with a plate-shaped base, so it has been necessary to develop a cap-shaped lid having a recessed interior for use with a plate-shaped base.
A cap-shaped lid having a flange which has been proposed in the past for use with a plate-shaped base is shown in FIGS. 6 and 7. FIG. 6 is a perspective view of a cap-shaped lid with a flange, and FIG. 7 is a vertical cross-sectional view of the lid of FIG. 6 sealed to a plate-shaped base to form a package for an electronic device. A cap-shaped lid 11 has a rectangular top surface 12 and four side walls 13 which are bent downwards from the top surface 12. The lower ends of the side walls 13 are bent sideways to form a flange 14. Solder 15 is adhered to the entire is back surface of the flanged cap-shaped lid 11.
An electronic device S such as a surface acoustic wave device is mounted on a plate-shaped base K and is electrically connected to electrodes on the base K through bumps B. The cap-shaped lid 11 is disposed on the base K so as not to contact the electronic device S, and in this state, the base K and the lid 11 are heated in heating apparatus such as a reflow furnace. The solder 15 on the back surface of the lid 11 melts in the furnace. Thereafter, as the solder 15 cools and solidifies, it seals the lid 11 and the base K to each other.
As electronic equipment becomes increasingly smaller, there is a demand for reduction in size of each electronic part used in such electronic equipment. In an electronic part sealed in a package comprising a plate-shaped base and a flanged, cap-shaped lid as shown in FIGS. 6 and 7, the base must include a portion which is soldered to the flange of the lid. This portion of the base does not in any way contribute to the function of the electronic part, so it makes the electronic part unnecessarily large and it interferes with reduction in size of electronic equipment.
For this reason, it has been proposed to employ a cap-shaped lid without a flange in combination with a plate-shaped base. FIG. 8 is a perspective view of such a cap-shaped lid 16 without a flange, and FIG. 9 is a vertical cross-sectional view of the lid 16 of FIG. 8 soldered to a plate-shaped base K on which an electronic device S is mounted. In these figures, the same portions as in FIGS. 6 and 7 are affixed with the same reference numbers or letters, so an explanation of those portions will be omitted. Although the lid 16 of FIGS. 8 and 9 is smaller than the lid 11 of FIGS. 6 and 7 because it does not include a flange, it has the problem that it is difficult to form a complete seal between the lid 16 and the base K. Namely, when the lid 16 of FIGS. 8 and 9 is disposed on the base K and the two are heated in a reflow furnace, since solder is not present in the regions where the side walls 13 of the lid 16 contact the base K, solder does not penetrate between the side wall 13 and the base K, so they may not be completely sealed to each other.